In the gasification of coal and the degasification thereof, a liquid (gas water) is recovered in the form of a gas-containing phenolic aqueous system by condensation of the liquids from the coal gases and/or by the washing thereof with water.
It is known to extract phenols from the aqueous phase produced in coal degassing the pressurized degasification of coal, with an extracting solvent and to strip the solvent with steam or gas. The steam or gas used to strip the solvent can be subjected to condensation with cold water or to washing with water and/or crude phenol, to eliminate solvent which may be entrained in the released gases (H. J. Wurm, Gluckauf 104, 12 (1968), 517-523).
From the literature it is also known to recover the solvent separately or in combination with an acid-gas stripping. Thus all of the strippable substances, for example, NH.sub.3, CO.sub.2, H.sub.2 S (acid gas) a portion of the HCN, inert gases and residual solvent may be driven off with steam at standard pressure substantially completely.
The conventional process is generally carried out as follows:
The dephenolated gas water resulting from the extraction is subjected to an after-precipitation of solvent and thereafter to a total stripping before being subjected to collection and batch treatment, e.g. in a waste-water-treatment plant or other installation capable of purifying this water. The clarified water may then be used for other purposes, e.g. as washing water at an earlier stage in the process.
The stripped vapors are generally then treated with sulfuric acid to produce ammonium sulfate and residual gas, consisting primarily of CO.sub.2, H.sub.2 S, HCN, steam, inert gases and residual solvent, can then be recovered during a subsequent solvent-recovery stage.
The advantage of this process is that it is not necessary to strip the solvent before the ammonia is recovered as was the case earlier because the solvent appeared in the vapor phase with the ammonia and the other volatile components. With the last-mentioned improvement the solvent recovery can result from a treatment of the vapors with crude phenol whereby the crude phenol acts as an extracting solvent to pick up the dephenolating solvent. The dephenolating solvent is separated from the phenol by distillation for further use.
This process has, however, the disadvantage that HCN is recirculated in the process and builds up in increasing concentrations in the phenol cycle. As a result there is undesirable corrosion, the formation of Prussian blue and other iron cyanide compounds which cannot block corrosion in systems which switch from acidic to basic characteristics during operation. Such switchover from acidic to basic characteristics frequently occurs in the conventional system because the phenolic liquids are acidic and the ammoniacal liquids are basic.
Furthermore, the conventional process gives rise to ammonium sulfate which is not always the most desirable ammonia-containing product. There are other processes for the treatment of the ammonia which is produced, e.g. involving a combination. This is also undesirable.
Even the recovery of ammonia in the form of ammoniacal water poses a problem because of possible losses of solvent. It is desirable, therefore, to recover the ammonia in a high-purity liquefied or gaseous state without traces of the dephenolated solvent.
For the recovery of pure ammonia several methods have been described using, for example, distillation-type separations, chemical washing or ion-exchange washing. The distillative method must be preceded by a solvent-removal step while the chemical and ion-exchange washing techniques make use of a scrubbing stage, a stripping stage and a regenerating stage through which the washing agent is circulated.
In all of the aforementioned techniques two main disadvantages arise, namely, the progressive enrichment in HCN of the solvent or solvents or other agents circulated in the process and the uneconomical character of the ammonium sulfate forming step.